1. Field of the Invention
The invention relates generally to the field of well logging instruments. More specifically, the invention relates to flexible couplings used to enable lateral displacement of well logging instrument housings relative to one another.
2. Background Art
Well logging instruments are used to make measurements of physical properties of earth formations from within wellbores drilled through such formations. Some of these instruments, such as neutron porosity measuring devices, make more precise measurements when placed into contact with the wall of the wellbore. Other types of well logging instruments, such as electromagnetic induction resistivity measuring devices, make more precise measurements when radially spaced apart from the wellbore wall by a predetermined distance. Still other types of well logging devices include small sensors which may be placed in contact with the wellbore wall over only a small portion of the circumference of the wellbore wall and over a very short axial length along the wall. These small sensors are generally placed in devices referred to as “pads” or “skids”, which selectably extend from the instrument housing to contact the wellbore wall. Common types of pad sensor logging instruments include backscatter-type gamma-gamma density sensors and various forms of very small-scale, or “micro”, resistivity devices. As is known in the art, a pad mounted sensor usually includes various linkages for causing the pad to selectably extend from the main housing of the logging instrument which place the pad in firm contact with the wall of the wellbore.
It is generally desirable, for reasons of economy of operation, to assemble as many different types of well logging sensors as is practical together in a single instrument assembly (called a “string”) so that many different types of measurements can be made in a single operation of the instrument string in the wellbore. As more logging sensors are assembled in the instrument string, operating the string becomes increasingly difficult, particularly because some of the instruments can be pad-type, others can be intended to be put in contact with the wellbore wall, and still others on the same string need to be separated from the wellbore wall. Conventional logging instruments typically are long enough so that natural flexibility in the instrument housings enables the different types of instruments to be placed in their proper radial positions with respect to the wellbore wall. If conventional logging instruments are used, however, the overall length of the instrument string can become so great as to materially increase the risk of the instrument string becoming stuck in the wellbore, among other hazards.
It is also known in the art to deploy well logging instruments through the interior of a drill string using a particular type of drill bit at the bottom of the drill string that has a selectably releasable closure element. One such system is disclosed in U.S. Pat. No. 6,269,891 issued to Runia.
U.S. Pat. No. 5,808,191 issued to Alexy et al. describes well logging instruments coupled end to end in which a device disposed between the instruments enables lateral displacement of one instrument relative to the other.
A particular design challenge in making and using devices such as those disclosed in the Alexy et al. '191 patent is providing a passage for electrical conductors and related items between the two well logging instruments. As is known in the art, the interior of a typical well logging instrument defines a sealed chamber in which electronic and other components disposed therein are maintained at surface atmospheric pressure. Thus, the device disclosed in the Alexy et al. '191 patent includes an internal passage that is sealed against fluid intrusion from outside the device while enabling relative axial displacement between the two ends of the device. The passage requires a seal mechanism that enables the described relative axial displacement. As will be appreciated by those skilled in the art, seals that enable relative motion between components of an instrument in a wellbore are inherently less reliable than seals that join instruments that are positionally fixed with respect to each other.
It is desirable to have a flexible coupling for well logging instruments that does not require sealing engagement between components able to more relative to one another.